I have a 30 year old bike (forgot the brand) and me and my friend were riding along and suddely my friend did X and we had to pedal real fast to get away. I stood up on the pedals and suddenly BOOM and I whack my leg real bad on the effing frame. We get away and I check it real quick and nothing seemed to be wrong. Then the chain got jammed in the back and came off in the front right in front of like 10 cars so I had to stop right there and fix the chain. Does anyone know why my bike is acting like this?